Neurons
Neurons or nerve cells are excitable cells in the nervous system that respond to stimuli. They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. Neurons do not go through mitosis, and usually cannot be replaced after being destroyed.
Neurons are highly specialized for the processing and transmission of cellular signals. Their wide variety in shape, size and electrochemical properties is reflective of the diversity of functions they perform in different parts of the nervous system. A number of specialized types of neurons exist. For example, sensory neurons (afferent) respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain, while motor neurons (efferent) receive signals from the brain and spinal cord, cause muscle contractions and affect glands.
The typical neuron has four morphologically defined regions: (1) the cell body (also called the soma or perikaryon), (2) the dendrites, (3) the axon and (4) the presynaptic terminals of the axon. Nerve cells generate active electrical signals, and each region has distinctive signaling functions.
The cell body is the metabolic center of the neuron. Three organelles are characteristic of the cell body: the nucleus, which in neurons often is quite large; the endoplasmic reticulum, upon which membrane and secretory proteins are synthesized; and the Golgi apparatus, which carries out the processing of secretory and membrane components. The cell body usually gives rise to several fine arborizing outgrowths or extensions called dendrites, which serve as the chief receptive apparatus for the neuron. The cell body also gives rise to the axon, a tubular process that can extend considerable distances (up to 1 m in humans).
The axon constitutes the conducting unit of the neuron. Axons lack ribosomes and therefore cannot synthesize proteins. Newly synthesized macromolecules are assembled into organelles within the cell body and are moved along the axon to the presynaptic terminals by a process called axoplasmic transport. When severed from the cell body, the axon usually degenerates and dies. Large axons are surrounded by a fatty insulating sheath called myelin, which is essential for high-speed conduction of action potentials. The myelin sheath is interrupted at very regular intervals. These points of interruption are called nodes of Ranvier.
Near its end, the axon divides into many fine branches, which have specialized endings called presynpatic terminals. The presynaptic terminals are the transmitting elements of the neuron. By means of its terminals, one neuron contacts and transmits information about its own activity to the receptive surfaces of another neuron, a muscle or other kinds of effector cells. The point of contact is the synapse. It is formed by the presynaptic terminal of one cell (the presynpatic cell), the receptive surface of the other (the postsynaptic cell), and the space separating them (the synaptic cleft). The terminals of the presynaptic neuron sometimes contact the postsynpatic neuron on its cell body, but more commonly the contacts occur on dendrites. Less often, synapses are located on the initial or on the terminal portions of axons.
On the basis of the number of processes that arise from the cell body, neurons are classified into three groups: unipolar, bipolar and multipolar.
Unipolar cells have one primary process that may give rise to many branches. Some branches serve as dendritic receiving structures, and other branches as axons and terminal structures. Unipolar cells predominate in the nervous systems of invertebrates in collections of nerve cells located near the spinal cord in the sensory ganglia of the dorsal roots.
Bipolar neurons have an ovoid soma that gives rise to one process at each end: a dendrite or peripheral process (which picks up information from the periphery), and an axon or central process (which carries information toward the central nervous system). The bipolar cells of the retina are examples of this class.
Multipolar neurons predominate in the vertebrate nervous system. These cells have one or more dendritic branches and a single axon. In a typical multipolar cell, dendrites emerge from all parts of the cell body; variants are the pyramidal cell of the cerebral cortex and the Purkinje cell, a class of GAGAergic neurons located in the cerebellar cortex.
Even within the category of multipolar neurons, the size and shape of different cells vary greatly. Different types of multipolar cells account for nearly all of the distinguishable neuronal cell types, which number between 1,000 and 10,000. The morphological differences among multipolar cells are due largely to variations in two features: the number and length of the dendrites, and the length of the axon. The number and extent of dendritic processes in a given cell correlate with the number of synaptic contacts that other neurons make on that cell. For example, a spinal motor cell, whose dendrites are moderate in both number and extent, receives roughly 10,000 contacts, while the large dendritic tree of the Purkinje cell of the cerebellum receives approximately 150,000 contacts.
The length of the axon reflects the signaling function of a neuron. Neurons with long axons (“Golgi type I cells”) carry information from one brain region to another; they serve as projection or relay neurons. Neurons with short axons (“Golgi type II cells”) primarily process information within a small, limited region of the brain. These nerve cells serve as local interneurons in various nuclei of the brain and in reflex pathways.
The movement of axons is determined by their growth cones, expansions of the tip of the growing axon that generate the mechanical force that pulls the axon forward. The growth cone has a broad sheet-like extension (lamellipodia) which contain protrusions (filopodia). The motility of growth cones is punctuated by cycles of protrusion, adhesion, and contraction. Actin plays a major role in the mobility of this system.
The direction pursued by the growth cones of an outgrowing axon is influenced by a variety of cues that range from (1) simple differences in the texture and stickiness of the substrate, to (2) rather precise molecular cues from recognition molecules imbedded in the surface membrane of the cells over which the growth cone crawls, to (3) diffusible gradients set up by a distant source. Further, guidepost cells, which typically are other neurons, may assist in the guidance of neuronal axon growth.
It is likely that the physical substrate over which an axon grows out contributes cues that guide the growth cone to its target. Substrates vary in adhesiveness, and that selective adhesion can guide the direction of the outgrowing process. Variations in the texture and shape of the available surfaces on which processes grow may produce regional differences in the adhesion between the growth cone and the substrate that determine the direction of growth. Environments with high levels of cell adhesion molecules (CAM's) create an ideal environment for axonal growth. Generally, it is believed that this provides a “sticky” surface along which axons can grow. Examples of CAM's specific to neural systems include the immunoglobulins N-CAM, neuroglial CAM (NgCAM), TAG-1, MAG, and DCC. Extracellular matrix adhesion molecules (ECMs) also provide a sticky substrate along which axons can grow. Examples of ECMs include laminin, fibronectin, tenascin, and perlecan. Some ECMs are surface-bound to cells and thus act as short range attractants or repellents. Others are diffusible ligands and thus can have long range effects.
In addition to adhesiveness, growth cones also might also sense more specific recognition molecules. For example, specific receptors on the surface of outgrowing growth cones might recognize molecules on the surface of cells forming the substrate. Alternatively, a signal molecule might be secreted by a substrate cell and internalized by the outgrowing cell; the molecule then could act from within the second cell to influence the direction of neurite outgrowth.
Nervous System
The nervous system is divided into two parts: the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of cranial and spinal nerves along with their associated ganglia.
Central Nervous System (CNS)
The CNS consists of six main regions: (1) the spinal cord; (2) the medulla; (3) the pons; (4) the midbrain; (5) the diencephalon; and (6) the cerebral hemispheres.
The spinal cord, the most caudal part of the CNS, receives information from the skin, joints, and muscles in the trunk and limbs, and it is the final way station for issuing commands for movement. In the spinal cord there is an orderly arrangement of motor and sensory nucleic controlling the limbs and trunk. In addition to nuclei, the spinal cord contains afferent pathways for sensory information to flow to the brain and efferent pathways for commands necessary for motor control to descend from the brain to the motor neurons. The spinal cord also receives sensory information from the internal organs and controls many autonomic functions.
The spinal cord continues rostrally as the brain stem, which comprises the medulla, the pons and the midbrain. The medulla is the most direct rostral extension of the spinal cord and resembles the spinal cord in aspects of its organization. The pons, which lies rostral to the medulla, contains a massive set of neurons that relay information from the cerebral hemispheres to the cerebellum. The cerebellum is not part of the brain stem, but because of its position dorsal to the pons, it is commonly grouped together with the pons. The midbrain lies rostral to the pons, and is important in the control of eye movement. The midbrain also contains an essential relay in the auditory pathway and several structures critically involved in motor control of skeletal muscles.
The diencephalon contains two key subdivisions: the thalamus and the hypothalamus. The thalamus processes and relays most of the information coming from the lower regions of the CNS en route to the cerebral cortex. The hypothalamus is important for integration in the autonomic nervous system and for regulating hormonal secretion by the pituitary gland.
The cerebral hemispheres consist of the cerebral cortex and the basal ganglia. Collectively termed the “cerebrum”, these structures are concerned with perceptual, cognitive, and higher motor functions. The cerebral cortex is further subdivided into four lobes: frontal, parietal, temporal and occipital. Large regions of the cerebral cortex are committed to movement and sensation. Areas that are directly committed are called primary, secondary, and tertiary sensory or motor areas. Surrounding the primary areas are higher order (secondary and tertiary) sensory and motor areas. These areas process more complex aspects of a single sensory modality or motor function than primary areas. The purpose of the higher order sensory areas is to achieve greater analysis and integration of information coming from the primary sensory areas. In contrast, the flow of information from the motor areas is in the opposite direction. Higher order motor areas distill complex information about a potential motor act and relay it to the primary motor cortex, which is the site from which voluntary movement is initiated.
Peripheral Nervous System (PNS)
The peripheral nervous system (PNS) resides or extends outside the CNS. The main function of the PNS is to connect the CNS to the limbs and organs. The PNS is divided by function into the somatic nervous system, the autonomic nervous system and the enteric nervous system.
The somatic nervous system is responsible for coordinating the body movements, and also for receiving external stimuli. It regulates activities that are under conscious control.
The autonomic nervous system (or autonomic motor system) provides the innervation for the endocrine and exocrine glands, for the viscera, and for smooth muscles in all organs of the body. The autonomic nervous system has two major divisions: the sympathetic and parasympathetic. The sympathetic system often mediates the response of the body to stress; it speeds up heart rate, increase blood pressure, mobilizes the body's energy stores for emergency, and prepares for action. In contrast, the parasympathetic system acts to conserve the body's resources and restore homeostasis; it slows the heart, reduces blood pressure, and prepares the body for relaxation and rest. The two divisions are segregated anatomically. The cell bodies that give rise to the sympathetic division lie in the thoracic and lumbar regions of the spinal cord. The neurons that give rise to the parasympathetic division lie above this region of the spinal cord in several brain stem nuclei associated with cranial nerves, and below it in the sacral region of the spinal cord. The autonomic nuclei in the brain stem and spinal cord contain neurons (preganglionic cells) that send their axons to synapse on a second set of neurons (postganglionic cells) that lie in peripheral collections of nerve cell bodies (autonomic ganglia). the postganglionic cells in turn innervate viscera, glands, and smooth muscle.
The main control center for the autonomic motor system is the hypothalamus, which also is critically involved in the regulation of feeding and drinking. The hypothalamus sends out descending fibers that regulate sympathetic and parasympathetic nuclei in the spinal cord and brain stem, axons that control the release of hormones by the anterior pituitary gland, and axons that release hormones directly into the posterior pituitary gland. The hypothalamus receives information from many other structures, including higher levels of the motivational systems: the cerebral cortex and the reticular formation.
The enteric nervous system controls the gastrointestinal system. The enteric nervous system is capable of autonomous functions, such as the coordination of reflexes, and may contain as many as 100,000,000 neurons. The neurons of the enteric nervous system are collected into two types of ganglia: myenteric (Auerbach's) and submucosal (Meissner's) plexuses. Myenteric plexuses are located between the inner and outer layers of the muscularis externa. Submucosal plexuses are located in the submucosa (the layer of dense irregular connective tissue that supports the mucosa). In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input.
Nerve Injury and Disorders
Damage to nervous tissue is particularly serious because most neurons in the adult mammalian CNS have withdrawn from the mitotic cycle and no longer are capable of cell division. Consequently, any physical injury that causes neurons to die will not be followed by regeneration of cells but will bring about a permanent change in the structure of the nervous system. This structural change usually is accompanied by long-lasting alterations in the functions of the affected areas.
The term “axotomy” refers to the cutting or severing of a neuron's axon. Cutting an axon interrupts both rapid axonal transport and slower axoplasmic flow, the two mechanisms that carry materials synthesized in the neuronal cell body to the axon terminals. The axon and the synaptic terminals degenerate when deprived of their normal metabolic interaction with the cell body. The term “anterograde” or “Wallerian” degeneration as used herein refers to degeneration of that part of the axon disconnected from the cell body which would be considered distal relative to the lesion. The term “retrograde” degeneration refers to changes proximal to the lesion site in the part of the axon that remains connected to the cell body. Retrograde changes are found quite frequently after axotomy; in some instances they are severe and can result in death of the neuron.
Synapses mediate not only electrical signals but also nutritive (trophic) interactions between neurons. Trophic factors are crucial for the normal maintenance of these cells. Like synaptic interactions, trophic interactions are thought to occur via a neuron's synaptic contacts. Deprived of its synaptic terminals, a neuron may shrink, atrophy or degenerate. Therefore, if a bundle of axons in the CNS is severed, degenerative changes may be found not only in the damaged neurons but also in neurons that receive synapses from the damaged neurons. In some injuries, the presynaptic neurons that synapse on the damaged cells also are affected (these reactions are referred to as “transsynpatic” or “transneuronal” meaning that they cross from one neuron to the next via the synapse). These influences may be mild, or they can be drastic and cause degeneration of the affected neurons. Transneuronal changes of various kinds are important in explaining why a lesion at one site in the CNS can have effects on sites distant to the lesion, sites that are distributed according to the connections that the lesion interrupts.
Glial Cells
In addition to neurons, nervous tissue contains glial cells (oligodendrocytes, astrocytes, ependymal cells, Schwann cells and microglia). Some of these cells play an important role in healing. Certain types of supporting cells absorb the cellular debris that results from neuronal injury by taking up and destroying (phagocytosing) toxic products of degeneration, while other supporting cells sometimes can interfere with healing if their proliferation blocks the restoration of severed synaptic connections within the brain and spinal cord. Therefore, the healing processes that are activated in the CNS by neuronal injury can be both helpful (e.g. phagocytosis) and troublesome (e.g. blocked regeneration).
Two types of glial cells, astrocytes and oligodendrocytes, vastly outnumber neurons. Astrocytes predominate in gray matter. They have small cell bodies (3-5 μm) that are packed with bundles of glial filaments about 100 nm in diameter. Numerous processes radiate from the cell body in various directions, and many of these come into close contact with blood vessels The term “sclerosis” often is used to describe disease states, such as multiple sclerosis, that affect populations of axons in the brain, and refers to the palpably hard scar of astrocytes that replaces phagocytosed debris resulting from the disease process.
Oligodendrocytes, which form myelin in the CNS, predominate in white matter. They have smaller cell bodies (1-3 μm in diameter) and give off fewer processes than astrocytes; each process appears to participate in forming myelin for a single axon. In the CNS, each oligodendrocyte contributes to the myelin sheath of several (as many as 20) axons by means of its different processes.
Glial cells proliferate around chromatolytic neurons and assume the appearance of phagocytes. The term “chromatolysis” (and its various grammatical forms) is used herein to refer to reorganizational changes in the cell body of a damaged neuron. Glial cells have been observed displacing presynaptic terminals along the proximal dendrites and cell bodies of axonotomized motor neurons. The pre- and postsynaptic elements of the synapse appear to be pushed apart by the invading glial cells. Damaged neurons receive reduced synaptic inputs, and the evoked excitatory postsynaptic potentials are smaller in amplitude, as if synapses on the cell body and proximal dendrites were removed by encroachment of glial cells. Even though somatic synapses are displaced, chromatolysing motor neurons still can be activated because remote synapses on their dendritic tree that normally are ineffective begin to excite the cell. After the normal input to the soma is removed, new trigger zones develop on the cell body and along the axon. A reorganization of this type may enable the cell to maintain normal number of synapses. If appropriate connections with muscles are established by the regenerating motor axons, then the normal input to the cell body of the motor neuron returns.
If a bundle of axons is cut, either by sectioning of a tract within the brain or by sectioning of a peripheral nerve, the site where the lesion is located is termed the “zone of trauma.” The part of the axon still connected to the cell body is the “proximal segment,” and the part isolated from the rest of the cell is the “distal segment.” The cut ends of both parts of the axon lose axoplasm immediately after injury, but the ends soon become sealed off by fusion of the axon membrane, retract from one another, and begin to swell. The swollen reaction bulbs that result are formed largely by materials carried along the axon by axonal transport and axoplasmic flow. Mitochondria, vesicles, multivesicular bodies and much unidentified membranous material pile up in the sealed end of each axon segment. Although both the proximal and the distal segments swell (because fast axonal transport occurs in two directions), the proximal end swells more, because newly synthesized neurofilaments, microtubules, and microfilaments, traveling by slow axoplasmic flow, come from the cell body only.
Types of Nerve Injury
Nerve injury may be classified into three types: neurapraxia; axonotmesis; and neurtmesis.
In neurapraxia, the least severe form of nerve injury with complete recovery, the actual structure of the nerve remains intact, but there is an interruption in conduction of the impulse down the nerve fiber. Most commonly, this involves compression of the nerve or disruption to the blood supply (ischemia). There is a temporary loss of function which is reversible within hours to months of the injury (the average is 6-8 weeks). Wallerian degeneration (a process that results when a nerve fiber is cut or crushed in which the part of the axon separated from the neuron's cell nucleus degenerates) does not occur, so recovery does not involve actual regeneration. There frequently is greater involvement of motor than sensory function with autonomic function being retained.
Axonotmesis, a more severe nerve injury with disruption of the neuronal axon, but with maintenance of the myelin sheath, results in loss of the relative continuity of the axon and its covering of myelin, but preservation of the connective tissue framework of the nerve (i.e, the encapsulating tissue, the epineurium and perineurium, are preserved) leading to Wallerian degeneration. Loss in both motor and sensory spines is more complete with axonotmesis than with neurapraxia, and recovery occurs only through regenerations of the axons. There usually is an element of retrograde proximal degeneration of the axon, and for regeneration to occur, this loss first must be overcome. The regeneration fibers must cross the injury site, and regeneration through the proximal or retrograde area of degeneration may require several weeks; then the neuritic tip progresses down the distal site. The proximal lesion may grow distally as fast as 2 mm to 3 mm per day and the distal lesion as slowly as 1.5 mm per day. Regeneration may take several weeks or years.
Neurotmesis, the most severe lesion with potential of recovery, occurs on severe contusion, stretch, laceration or local anesthetic toxicity. Not only the axon but also the encapsulating connective tissue lose their connectivity. The last (extreme) degree of neurotmesis is transection. Most neurotmetic injuries do not produce gross loss of continuity of the nerve but rather internal disruption of the architecture of the nerve sufficient to involve the perineurium [one of the supporting structures of peripheral nerve trunks, consisting of layers off lattened cells and collagenous connective tissue, which surround the nerve fasciculi and form the major diffusion barrier within the nerve] and endoneurium [the innermost connective tissue supportive structure of nerve trunks that surrounds both myelinated and unmyelinated nerve fibers, consisting principally of ground substance, collagen, and fibroblasts] as well as axons and their covering. There is a complete loss of motor, sensory and autonomic function. If the nerve has been completely divided, axonal regeneration causes a neuroma (swelling or pseudoneuroma) to form in the proximal stump.
Neuroregeneration
The term “neuroregeneration” (or “nerve regeneration”) refers to the growth or repair of nervous tissues, cells or cell products. Repair mechanisms may include, but are not limited to, remyelination and generation of new neurons, glia, axons, myelin and synapses.
While the PNS has an intrinsic ability for repair and regeneration, the CNS is, for the most part, incapable of self-repair and regeneration. Currently, there is no treatment for recovering human nerve function after injury to the CNS.
Neuroregeneration in the CNS
Unlike PNS injury, injury to the CNS is not followed by extensive regeneration. Several factors may contribute to this inactivity. The environment within the CNS, especially following trauma, hinders the repair of myelin and neurons, and, generally, growth factors are not expressed or re-expressed (for example, the extracellular matrix lacks laminins). Additionally, the axons themselves lose the potential for growth with age. Further, a distal segment in the CNS degenerates slower than in the PNS; the slower removal of myelin and axonal debris contributes to the inhibitory environment. All these factors contribute to the formation of what is known as a glial scar, across which axons cannot grow. Several families of molecules are released that promote and drive glial scar formation. Transforming growth factors β-1 and β-2, interleukins, and cytokines all are believed to play a role in the initiation of scar formation. The glia further produce factors that inhibit remyelination and axon repair, such as, for example, NOGO and NI-35.
At a zone of trauma in the CNS, the axon and myelin sheath undergo rapid local degeneration. Because blood vessels usually are interrupted by a lesion, macrophages from the general circulation can enter the area and phagocytose axonal debris. Glial cells (astrocytes and microglia) also proliferate and act as phagocytes. In the CNS, however, the proliferation of fibrous astrocytes leads to the formation of a glial scar around the zone of trauma. Scarring can block the course taken by regenerating axons and establish an effective barrier against the reformation of central connections.
Degeneration spreads in both directions along the axon from the zone of trauma, but only for a short distance in the proximal segment, usually up to the point of origin of the first axon collateral. After 2-3 days, a retrograde reaction is seen in the cell body. If the entire cell body dies, then degeneration spreads from the axon hillock (the conical area of origin of the axon from the nerve cell body) down along the remainder of the proximal segment. In the distal segment, outside the zone of trauma, degeneration first appears in the axon terminal about 1 day after the occurrence of the lesion. In approximately 2 weeks, the synapses formed by the distal segment degenerate completely (terminal degeneration). Degeneration of the distal axon itself takes place over a period of 1-2 months (Wallerian degeneration). Eventually, cells that are either pre- or postsynaptic to the injured neuron also may be affected (“transneuronal degeneration”). Thus, in anterograde transneuronal degradation, neurons deprived of major input from axons that have been destroyed may atrophy. In retrograde transneuronal degradation, similar changes may occur in neurons that have lost the main recipient of their outflow.
The axon terminal is very sensitive to interruption of contact with the parent cell body. If the axon of a motor neuron to a skeletal muscle is severed by cutting a peripheral nerve, within a matter of hours degenerative changes begin to occur at the presynaptic terminals of the motor axon because the maintenance of its integrity is critically dependent on fast axonal transport. Synaptic transmission fails soon after the axon is cut, even before the first morphological signs of degeneration become evident in the synaptic terminal. The onset of transmission failure is very rapid if the axon is cut close to the synaptic terminal region, and slower if the axon is cut close to the cell body. This indicates that axonal transport continues for some time in the distal segment until the entire axon is depleted of metabolic products required for synaptic transmission.
The degenerative changes that occur in the synaptic terminal itself are similar to the changes that take place in degenerating synapses in the CNS. Within one day after axotomy, the terminal and its mitochondria begin to swell. In some cases the terminal becomes filled with swirls of neurofilaments surrounding a central packet of disrupted mitochondria. Alternatively, the terminal may become filled with more homogeneous electron-dense products of degeneration. After 6 or 7 days the terminal is pushed away from its contacts with postsynaptic neurons by invading glial cells. At the neuromuscular synapse, eventually the Schwann cells around the synaptic terminal of the motor axon de-differentiate and proliferate to form phagocytes that absorb the degenerating terminal. Soon afterward, the whole distal axon breaks up into short, beaded segments that then are phagocytosed by Schwann cells.
About one week after the initial degenerative changes appear in the axon terminal, degeneration begins in the entire distal axon. The myelin sheath draws away from the axon and breaks apart. The axon swells and then becomes beaded. Neurofilaments and neurotubules (collectively neurofibrils) soon fill the axon. Fragments of the axon and the myelin sheath are absorbed by local phagocytes derived from the glial cell population in the CNS or from Schwann cells in the PNS. In the CNS, macrophages from the general circulation do not absorb the debris produced by Wallerian degeneration, as they do in the zone of trauma.
Neuroregeneration in the PNS
Neuroregeneration in the PNS occurs to a significant degree. Injury to the PNS immediately elicits the migration of phagocytic cells, Schwann cells, and macrophages to the lesion site in order to clear away debris, such as damaged tissue. After injury, the proximal end swells and experiences some retrograde degeneration, but once the debris is cleared, it begins to sprout axons and the presence of growth cones can be detected. The proximal axons are able to regrow as long as the cell body is intact, and they have made contact with the neurolemmocytes in the endoneurial channel. Human axon growth rates can reach 2 mm per day in small nerves and 5 mm per day in large nerves. The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium (a delicate connective tissue around individual nerve fibers in a nerve bundle) remains. In the later stages of regeneration, the remaining endoneurial tube directs axon growth back to the correct targets. During Wallerian degeneration, Schwann cells grow in ordered columns along the endoneurial tube, creating a band of Bungner (boB) that protects and preserves the endoneurial channel. Also, macrophages and Schwann cells release neurotrophic factors that enhance re-growth.
The sequence of axonal degeneration in the PNS differs from the sequences that occurs in the CNS. If the peripherally directed process of a dorsal root ganglion cell is cut, or if a motor axon is cut, then the distal segment of the severed axon will degenerate. However, the connective tissue sheath that surrounds the nerve in which the severed axon ran may remain intact. In many instances, depending upon the nature of the injury, the proximal segment of a severed axon can regenerate and reconnect to its previous synaptic sites as long as its cell body remains alive. The regenerating axons run along the connective tissue sheath, which acts as a conduit leading the growing axons back to the peripheral target. Conversely, if the centrally directed branches of dorsal root ganglion cells are cut, the glial scar that forms around the degenerating axons in the dorsal aspect of the spinal cord prevents any axons that might regenerate from reaching their central targets.
There are two major ways in which the cell bodies of different classes of neurons respond to axotomy. After an axon is severed, some neurons undergo distinctive regenerative changes as they prepare metabolically for the regrowth of a new axon. For example, cutting the peripheral axon of a dorsal root ganglion cell or a spinal motor neuron causes characteristic changes in the parent neuron within 2-3 days. The cell body first begins to swell (it may double in size). The nucleus moves to an eccentric position, usually opposite the axon hillock, and also begins to swell. Finally, the rough endoplasmic reticulum (ER) breaks apart and moves to the periphery of the swollen cell body. For 1-3 weeks, the number of free polysomes in the cell body, the total amount of protein, and RNA synthesis in the nucleus increases (chromatolysis), suggesting that a massive synthesis of proteins necessary for regenerating the severed parts of the axon occurs. If the proper connections are restored after regeneration of the axon, this buildup ceases and the cell body usually regains its normal appearance. If the proper connections are not restored, the cell will atrophy or degenerate totally. The age of the animal, the site of the lesion, and the nature of the injury are important considerations in judging the potential for functional recovery after nerve section.
Neuroprotection
Neuroprotection refers to the mechanisms and/or strategies used to guard or defend against neuronal injury or degeneration in the CNS following acute disorders (such as, for example, stroke, nervous system injury or trauma) or as a result of chronic neurodegenerative diseases (such as, for example, Parkinson's disease, Alzheimer's disease, Multiple Sclerosis). Neuroprotectives (products or compounds with neuroprotective effects) can be grouped into several categories including, but not limited to, the following: free radical scavengers; anti-excitotoxic agents; apoptosis inhibitors; anti-inflammatory agents; neurotrophic factors; metal ion chelators; and ion channel modulators.
Free Radical Scavengers
A free radical is a highly reactive and usually short-lived molecular fragment with one or more unpaired electrons. Free radicals are highly chemically reactive molecules. Because a free radical needs to extract a second electron from a neighboring molecule to pair its single electron, it often reacts with other molecules, which initiates the formation of many more free radical species in a self-propagating chain reaction. This ability to be self-propagating makes free radicals highly toxic to living organisms.
Reactive oxygen species (“ROS”), such as free radicals and peroxides, represent a class of molecules that are derived from the metabolism of oxygen and exist inherently in all aerobic organisms. The term “oxygen radicals” as used herein refers to any oxygen species that carries an unpaired electron (except free oxygen). The transfer of electrons to oxygen also may lead to the production of toxic free radical species. The best documented of these is the superoxide radical. Oxygen radicals, such as the hydroxyl radical (OH—) and the superoxide ion (O2-) are very powerful oxidizing agents that cause structural damage to proteins, lipids and nucleic acids. The free radical superoxide anion, a product of normal cellular metabolism, is produced mainly in mitochondria because of incomplete reduction of oxygen. The superoxide radical, although unreactive compared with many other radicals, may be converted by biological systems into other more reactive species, such as peroxyl (ROO—), alkoxyl (RO—) and hydroxyl (OH—) radicals.
Oxidative injury may lead to widespread biochemical damage within the cell. The molecular mechanisms responsible for this damage are complex. For example, free radicals may damage intracellular macromolecules, such as nucleic acids (e.g., DNA and RNA), proteins, and lipids. Free radical damage to cellular proteins may lead to loss of enzymatic function and cell death. Free radical damage to DNA may cause problems in replication or transcription, leading to cell death or uncontrolled cell growth. Free radical damage to cell membrane lipids may cause the damaged membranes to lose their ability to transport oxygen, nutrients or water to cells.
Free radical scavengers with a neuroprotective effect include, but are not limited to, 3-methyl-1-phenyl-2-pyrazolin-5-one (edaravone), and α-phenyl-n-tert-butyl-nitrone (PBN), N-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN).
Anti-Excitotoxic Agents
Excitatory acidic amino acids (EAAS) constitute the major group of exitatory neurotransmitters in the mammalian brain. They serve a multitude of defined physiological functions, which are the subject of several studies. It generally is believed that EAAS play a critical role in neuronal development, learning processes and motor control. Their actions are mediated by membrane receptors, which are classically divided into three pharmacologically distinct subtypes: N-methyl-D-aspartate (NMDA), quisqualate, and kainate receptors. Further, EAAS can produce selective “axon-sparing” neuronal lesions in the CNS. The term “excitotoxicity” refers to the pathological process by which nerve cells are damaged and killed by glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate (glutamate receptors (NMDA receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA or quisqualate receptor))) are overactivated. Excitotoxins, such as, but not limited to, NMDA and kainic acid, which bind to these receptors, and pathologically high levels of glutamate, can cause excitotoxicity by allowing high levels of calcium ions (Ca2+) to enter the cell. Ca2+ influx into cells activates a number of enzymes, including, but not limited to, phospholipases, endonucleases, and proteases, such as, for example, calpain. These enzymes go on to damage cell structures including, but not limited to, components of the cytoskeleton, the cell membrane, and DNA. Anti-excitotoxic agents include, but not are limited to, NMDA antagonists, phencyclidine, ketamine, (±)-SKF 10,047, pentazocine, d-aminophosphonovalerate, d-aminophosphonoheptanoate, d-α-aminoadipate, OH-quinoxaline carboxylate, kynurenate, (±)-cis-2,3-piperidine dicarboxylate, secobarbital, amobarbital and pentobarbital.
Apoptosis Inhibitors
The terms “apoptosis” or “programmed cell death” refer to a highly regulated and active process that contributes to biologic homeostasis comprised of a series of biochemical events that lead to a variety of morphological changes, including blebbing, changes to the cell membrane, such as loss of membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation, without damaging the organism.
Apoptotic cell death is induced by many different factors and involves numerous signaling pathways, some dependent on caspase proteases (a class of cysteine proteases) and others that are caspase independent. It can be triggered by many different cellular stimuli, including cell surface receptors, mitochondrial response to stress, and cytotoxic T cells, resulting in activation of apoptotic signaling pathways
The caspases involved in apoptosis convey the apoptotic signal in a proteolytic cascade, with caspases cleaving and activating other caspases that then degrade other cellular targets that lead to cell death. The caspases at the upper end of the cascade include caspase-8 and caspase-9. Caspase-8 is the initial caspase involved in response to receptors with a death domain (DD) like Fas.
Receptors in the TNF receptor family are associated with the induction of apoptosis, as well as inflammatory signaling. The Fas receptor (CD95) mediates apoptotic signaling by Fas-ligand expressed on the surface of other cells. The Fas-FasL interaction plays an important role in the immune system and lack of this system leads to autoimmunity, indicating that Fas-mediated apoptosis removes self-reactive lymphocytes. Fas signaling also is involved in immune surveillance to remove transformed cells and virus infected cells. Binding of Fas to oligimerized FasL on another cell activates apoptotic signaling through a cytoplasmic domain termed the death domain (DD) that interacts with signaling adaptors including FAF, FADD and DAX to activate the caspase proteolytic cascade. Caspase-8 and caspase-10 first are activated to then cleave and activate downstream caspases and a variety of cellular substrates that lead to cell death.
Mitochondria participate in apoptotic signaling pathways through the release of mitochondrial proteins into the cytoplasm. Cytochrome c, a key protein in electron transport, is released from mitochondria in response to apoptotic signals, and activates Apaf-1, a protease released from mitochondria. Activated Apaf-1 activates caspase-9 and the rest of the caspase pathway. Smac/DIABLO is released from mitochondria and inhibits IAP proteins that normally interact with caspase-9 to inhibit apoptosis. Apoptosis regulation by Bcl-2 family proteins occurs as family members form complexes that enter the mitochondrial membrane, regulating the release of cytochrome c and other proteins. TNF family receptors that cause apoptosis directly activate the caspase cascade, but can also activate Bid, a Bcl-2 family member, which activates mitochondria-mediated apoptosis. Bax, another Bcl-2 family member, is activated by this pathway to localize to the mitochondrial membrane and increase its permeability, releasing cytochrome c and other mitochondrial proteins. Bcl-2 and Bcl-xL prevent pore formation, blocking apoptosis. Like cytochrome c, AIF (apoptosis-inducing factor) is a protein found in mitochondria that is released from mitochondria by apoptotic stimuli. While cytochrome C is linked to caspase-dependent apoptotic signaling, AIF release stimulates caspase-independent apoptosis, moving into the nucleus where it binds DNA. DNA binding by AIF stimulates chromatin condensation, and DNA fragmentation, perhaps through recruitment of nucleases.
The mitochondrial stress pathway begins with the release of cytochrome c from mitochondria, which then interacts with Apaf-1, causing self-cleavage and activation of caspase-9. Caspase-3, -6 and -7 are downstream caspases that are activated by the upstream proteases and act themselves to cleave cellular targets.
Granzyme B and perforin proteins released by cytotoxic T cells induce apoptosis in target cells, forming transmembrane pores, and triggering apoptosis, perhaps through cleavage of caspases, although caspase-independent mechanisms of Granzyme B mediated apoptosis have been suggested.
Fragmentation of the nuclear genome by multiple nucleases activated by apoptotic signaling pathways to create a nucleosomal ladder is a cellular response characteristic of apoptosis. One nuclease involved in apoptosis is DNA fragmentation factor (DFF), a caspase-activated DNAse (CAD). DFF/CAD is activated through cleavage of its associated inhibitor ICAD by caspases proteases during apoptosis. DFF/CAD interacts with chromatin components such as topoisomerase II and histone H1 to condense chromatin structure and perhaps recruit CAD to chromatin. Another apoptosis activated protease is endonuclease G (EndoG). EndoG is encoded in the nuclear genome but is localized to mitochondria in normal cells. EndoG may play a role in the replication of the mitochondrial genome, as well as in apoptosis. Apoptotic signaling causes the release of EndoG from mitochondria. The EndoG and DFF/CAD pathways are independent since the EndoG pathway still occurs in cells lacking DFF.
Hypoxia, as well as hypoxia followed by reoxygenation can trigger cytochrome c release and apoptosis. Glycogen synthase kinase (GSK-3) a serine-threonine kinase ubiquitously expressed in most cell types, appears to mediate or potentiate apoptosis due to many stimuli that activate the mitochondrial cell death pathway. Loberg, R D, et al., J. Biol. Chem. 277 (44): 41667-673 (2002). It has been demonstrated to induce caspase 3 activation and to activate the proapoptotic tumor suppressor gene p53. It also has been suggested that GSK-3 promotes activation and translocation of the proapoptotic Bcl-2 family member, Bax, which, upon agregation and mitochondrial localization, induces cytochrome c release. Akt is a critical regulator of GSK-3, and phosphorylation and inactivation of GSK-3 may mediate some of the antiapoptotic effects of Akt.
It generally is believed that apoptosis contributes to neuronal cell death in a variety of neurodegenerative contexts. Activation of cysteine protease caspase-3 appears to be a key event in the execution of apoptosis in the CNS. Caspase-3 activation has been observed in stroke, spinal cord trauma, head injury and Alzheimer's disease. Some studies have shown that peptide-based caspase inhibitors can prevent neuronal loss in animal models of head injury and stroke. Further, failed caspase inhibition may have a role in spinal muscular atrophy (SMA) (a hereditary neurodegenerative disorder). In severe SMA, the neuronal specific inhibitor of apoptosis (IAP) family member known as NAIP often is dysfunctional due to missense and truncation mutations. IAPs such as NAIP potently block the enzymatic activity of group II caspases (3 and 7); NAIP mutations may permit unopposed developmental apoptosis to occur in sensory and motor systems resulting in lethal muscular atrophy (see, for example, Robertson, G. S., et al., Brain Pathology. 2006. 10 (2):283-292). Neuroprotective apoptosis inhibitors include, but are not limited to, boc-aspartyl(Ome)-fluoromethylketone, erythropoietin, and (R,S)-({(2S)-2-[5-tert-butyl-3-{[(4-methyl-1,2,5-oxadiazol-3-yl)methyl]amino}-2-oxopyrazin-1(2H)-yl]butanoyl}amino)-5-[hexyl(methyl)amino]-4-oxopentanoic acid bis-hydrochloride (M826).
Anti-Inflammatory Agents
A sustained inflammatory reaction is present in acute neurodegenerative disorders (such as, for example, stroke) and chronic neurodegenerative disorders (such as, for example, Alzheimer's disease, Parkinson's disease and multiple sclerosis). Inflammation, which is fostered by both residential glial cells and blood-circulating cells that infiltrate the diseased brain, probably starts as a time- and site-specific defense mechanism that could later evolve into a destructive and uncontrolled reaction. An acute neuroinflammatory response includes activation of microglia, resident tissue macrophages in the CNS and the principle mediators of neuroinflammation, resulting in phagocytosis and the release of inflammatory mediators such as cytokines and chemokines. Chronic neuroinflammation includes long-standing activation of microglia and subsequent sustained release of inflammatory mediators, which perpetuate the inflammatory cycle, activating additional microglia, promoting their proliferation, and resulting in further release of inflammatory factors. Several anti-inflammatory agents are generally believed to provide a neuroprotective effect including, but not limited to, non-steroidal anti-inflammatory drugs (NSAIDS), such as, but not limited to, aspirin, ibuprofen, indomethacin, sulindac, and flurbiprofen; estrogen; and peroxisome proliferator-activated receptor-γ (PPARs) agonists, such as, but not limited to, thiazolidinediones (TZDs).
Neurotrophic Factors
Neurotrophic factors are important regulators of the development and maintenance of vertebrate nervous systems. Neurotrophins are a unique family of polypeptide growth factors that influence the proliferation, differentiation, survival, and death of neuronal and nonneuronal cells. The effects of neurotrophins depend upon their level of availability, their binding affinity to transmembrane receptors, and the downstream signaling cascades that are stimulated after receptor activation. Neurotrophins have multiple roles in the adult nervous system including, but not limited to, regulating synaptic connections and synapse structure, neurotransmitter release and potentiation, mechanosensation, and pain and synaptic plasticity.
Many growth factors and neurotrophins can promote neuronal survival. These factors can activate several intracellular signaling transduction systems including, but not limited to, the extracellular signal-regulated kinase (ERK) and the phosphatidyl-inositol-3-OH kinase (PI 3-kinase) pathways. Studies have reported that activation of the PI 3-kinase pathway is required for (1) NGF-mediated survival of (a) the rat pheochromocytoma cell line PC12 (Greene and Tischler, 1976) (an in vitro cell culture system for studying the mechanism of NGF action) and (b) rat superiorcervical ganglion (SCG) neurons; (2) insulin-like growth factor-1-mediated survival of (a) cerebellar granule neurons, (b) oligodendrocytes, and (c) PC12 cells; and (3) for membrane depolarization-mediated survival of cerebellar granule neurons. Neuroprotective neurotrophic factors include, but are not limited to, brain-derived neurotrophic factor (BDNF), nerve-growth factor (NGF), neurotrophins 3 and 4/5, glial-derived neurotrophic factor (GDNF), and ciliary neurotrophic factor (CNTF).
Metal Ion Chelators
Metal ions are associated with metabolic processes (such as, for example, protein aggregation and oxidative stress) that are involved in several neurodegenerative disorders. Several chelators have been studied for their potential in the treatment of neurodegenerative diseases including, but not limited to, (1) hexadentate chelators, such as, for example, desferrioxamine, and a synthetic amino-carboxylate ligand (DP-109); (2) tridentate chelators, such as, for example, isonicotinoyl picolinoyl hydrazine; and (3) bidentate chelators, such as, for example, bathocuproine, feralex, and 8-hydroxyquinoline analogues.
One of the dominant properties of any therapeutic chelator is metal selectivity, typically a high selectivity being required (as with, for example, the treatment of iron overload associated with β-thalassaemia, where ligands with a high selectivity for iron over copper and zinc are essential, since chelation therapy is maintained for life). Unfortunately, the identity of the putative toxic metal is not always firmly established with many proposed treatments of neurodegenerative diseases by chelation therapy. For example, in Alzheimer's Disease, for instance, iron, copper and zinc all have been associated with the progression of the disease. Although there are clear guidelines for the design of iron-selective chelating agents (Liu & Hider, 2002), no clear guidelines exist for the design of copper and zinc selective chelating agents. Furthermore, because of the need for ready permeation of the blood-brain barrier (BBB), the size of useful chelators generally is limited to less than 300 Da, thereby excluding hexadentate ligands and seriously limiting the potential for the design of selective copper(II) and zinc(II) chelators. Any agent that binds copper(II) tightly also will bind iron(II), zinc(II), nickel(II), cobalt(II) and manganese(II), thereby causing a potential toxic insult to most cell types (Liu & Hider, 2002). This limitation is a major issue for the design of chelators potentially useful for treating neurodegeneration.
Ion Channel Modulators
Ion channels are pore-forming proteins that regulate the cell potential across the plasma membrane of all living cells; ion channels allow a flow of ions down their electrochemical gradient, i.e., from high concentration to low concentration. Ion channels are prominent components of the nervous system since “voltage-activated” channels underlie the nerve impulse, and “transmitter-activated” channels mediate conduction across the synapses. There are numerous types of ion channels that can be classified by gating (meaning by what opens and closes the channel) including (1) voltage-gated (ion channels that are reactive to membrane potential); (2) ligand-gated (ionotropic receptors that are reactive to specific ligand molecules); (3) ion gated (those channels reactive to ions such as Cl, K+, Na+, Ca2+) and (4) other gating (those reactive to, for example, second messengers). Several ion channel modulators with neuroprotective effects include, but not limited to, arachidonic acid, dantrolene, tetrodotoxin, polyamines, and estradiol.
Nerve Damage and Neuropathies
Neuron injury may result in several types of neuropathy.
Diabetic Neuropathies
Diabetic neuropathies are a family of nerve disorders caused by diabetes. Patients with diabetes may, over time, develop nerve damage throughout the body, while others may present no symptoms. Symptoms include pain, tingling, or numbness, in the hands, arms, feet, and legs. Nerve problems can occur in every organ system, including the digestive tract, heart, and sex organs. Proximal neuropathy results in pain in the thighs, hips, or buttocks and leads to weakness in the legs, and focal neuropathy results in the sudden weakness of one nerve or a group of nerves, causing muscle weakness or pain.
About 60%-70% of patients with diabetes have some form of neuropathy. The risk rises with age and longer duration of diabetes. The highest rates of neuropathy are among patients who have had diabetes for at least 25 years. Diabetic neuropathies also appear to be more common in patients who have problems controlling their blood glucose, those with high levels of blood fat and blood pressure, and those who are overweight.
Peripheral Neuropathy
Peripheral neuropathy, the most common type of diabetic neuropathy, also can result from traumatic injuries, infections, metabolic disorders and exposure to toxins. In its most common form, it causes pain and numbness in a subject's hands and feet. The pain typically is described as tingling or burning, while the loss of sensation often is compared to the feeling of wearing a thin stocking or glove. In many cases, peripheral neuropathy symptoms, when caused by a treatable underlying condition, improve with time. Medications initially designed to treat other conditions, such as epilepsy and depression, often are used to reduce the painful symptoms of peripheral neuropathy.
Autonomic Peripheral Neuropathy
Autonomic neuropathy is a form of peripheral neuropathy that involves damage to the nerves that run through a part of the PNS. It is a group of symptoms, not a specific disease, and has many causes. Symptoms occur when there is damage to nerves that regulate vital functions, including heart muscle, smooth muscles, those that regulate blood pressure, heart rate, bowel and bladder emptying, digestion, and other body functions. Autonomic neuropathy causes changes in digestion, bowel and bladder function, sexual response, perspiration, can affect nerves in the lungs and eyes, and may cause hypoglycemia unawareness, a condition in which patients no longer experience the warning symptoms of low blood glucose levels. Damage to the autonomic nerves also affects the function of areas connected to the problem nerve. For example, damage to the nerves of the gastrointestinal tract makes it harder to move food during digestion (decreased gastric motility).
CNS Nerve Degeneration
Damage to neurons of CNS may lead to progressive degenerative diseases.
For example, Alzheimer's disease (AD), a progressive, degenerative brain disease that affects memory, thinking, and behavior, is characterized by loss of neurons and synapses in the cerebral cortex and certain subcortical regions, which results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyms. Both amyloid plaques and neurofibrillary tangles, which are aggregates of the microtubule-associated protein tau, which has become hyperphosphorylated and accumulates inside the cells, are apparent. Although many older individuals develop some plaques and tangles as a consequence of aging, the brains of AD patients have a greater number of them in specific brain regions, such as the temporal lobe.
Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) belongs to a class of disorders known as motor neuron disorders in which the motor neurons located in the brain, brainstem and spinal cord that serve as the controlling units and are vital for communication links between the nervous system and the voluntary muscles of the body are affected. The loss of these cells causes the muscles under their control to weaken and waste away, leading to paralysis. It usually is fatal within five years of diagnosis. There is no cure for ALS, nor is there a proven therapy that will prevent or reverse its course. ALS affects 30,000 U.S. residents with about 5,000 new cases occurring in the U.S. each year. In about 10% of cases, ALS is caused by a genetic defect. In other cases, the cause of the nerve degeneration is unknown.
Parkinson's Disease
Parkinson's disease is a progressive disorder of the brain in which the nerve cells in the part of the brain that controls muscle movement gradually are destroyed. Symptoms include tremor and difficulty with walking, movement, and coordination. The exact reason that the cells of the brain waste away is unknown. The disorder may affect one or both sides of the body, with varying degrees of loss of function. The disease affects approximately 2 of every 1,000 people, both men and women, and most often develops after age 50. It may occur in younger adults, but is seen rarely in children.
Spinal Cord Injury
Spinal cord injury (SCI) involves damage to the nerves within the spinal canal; most SCIs are caused by trauma to the vertebral column, thereby affecting the spinal cord's ability to send and receive messages from the brain to the body's systems that control sensory, motor and autonomic function below the level of injury. Causes of paralysis include stroke, post-polio syndrome, cerebral palsy, neurofibromatosis, traumatic brain injury, spinal cord injury, multiple sclerosis, and unspecified birth defect. Various types of accidents accounted for the great majority of SCI.
The cost of living with spinal cord injury, which can be considerable, varies greatly depending on the severity of the injury. Average yearly expenses can range from $228,566 to $775,567 in the first year. The estimated lifetime costs due to SCI can range from $691,843 to over $3 million for a 25 year old. Further, 87.9% of all SCI individuals are discharged from hospitals to private homes.
Generally, clinical treatments for nerve injury are lacking, with any nerve regeneration being modest at best. Nerve autografting (or autologous nerve grafting) has been used to treat large lesion gaps in the PNS. The procedure involves transplanting nerve segments from a donor site within a subject to another (injured) site such that endoneurial tubes for axonal regeneration across the gap are provided. However, this treatment often provides only a limited functional recovery. Additionally, partial deinnervation frequently is experienced at the donor site and multiple surgeries are required to harvest the tissue and implant it.
Several variations of nerve autografting have been attempted. These include allografts (utilizing tissue from a donor that is implanted in the subject) and xenografts (utilizing tissue from a different species). Allografts and xenografts, in addition to having the disadvantages of autografts, often require simultaneous immunosuppressive therapies to mediate the recipient subject's immunological acceptance of the foreign tissue. Further, disease transmission must be considered when introducing tissue from another person or animal.
Additional efforts to effect nerve regeneration include the fabrication and use of nerve guidance conduits to guide axonal regrowth (where the artificial nerve conduits are introduced into the lesion) and immunization. However, these treatments also are lacking in effectiveness and may be costly.
While the efforts towards regenerating nerves of the PNS have yielded sparse, if any, results, there are no effective treatments for nerve injury or methods to facilitate nerve regeneration within the CNS.
The described invention addresses this problem. It provides and EPRO compositions comprising at least one peptide of formula I for improving or enhancing neurite outgrowth, neuroprotection, and nerve regeneration, and methods of use thereof.